For some reason I thought you couldn't replay the Most Wanted races, I'll give it a go on my lunchbreak cheers.
If you don't shut down the car after the race, you haven't unlocked it. You could just replay the Most Wanted race if you can't find the car any more.
Does anyone know what happens if you win a Most Wanted race but then dont't takedown the car afterwards? I got distracted after the race and it flew off into the distance! There isn't any marker on the map for it as far as Ic an tell.